Earthlings in the 20th century learned the value of putting multiple engines on aircraft: that way, if one fails, you can keep flying.
But in the Star Trek universe, Federation ships like the Enterprise have only one warp core. (AFAIK — I can't say I've examined every Federation ship before asking this question.)
Why is this so? If the warp core is so important, why wouldn't ships be built with two, so they have a backup? That way, if one fails, you wouldn't need to, oh I don't know, send key crew members into an irradiated area to fix it.
I understand that a second warp core would, in practice, deprive Star Trek of a whole mess of storylines. What I'm asking is whether this restriction has ever been explained in-universe.